Rotary position detectors have many uses, such as detecting the position of the shaft on a motor, for the purpose of electrical commutation. Another such use is to detect the position of a tensioner pulley in a magnetic tape player or web-type printing press, for the purpose of maintaining a constant tension on the tape or paper. One of the more recent uses of a rotary position detector is to sense the position of the accelerator (gas) pedal in an automobile that uses electric motors as a partial or complete means of driving the wheels.
Galvanometer-based optical scanners are used to direct non-moving input light beams to a target area. This type of scanner uses a limited-rotation motor to impart rotational motion onto an optical element, such as a mirror. Normally the mirror is mounted directly on the output shaft of the motor. A position detector is included within the motor, either close to the output shaft, or on the “rear” portion of the motor. This position detector normally outputs a current or voltage signal that is proportional to the relative angle of the motor shaft, and thus, relative to the angle of the mirror with respect to the non-moving input light beam.
Galvanometer-based optical scanners direct a laser beam for marking, cutting, or display purposes, for which positioning accuracy and repeatability can be of critical importance. Therefore, one of the limiting factors of accuracy and repeatability is the performance of the position detector used with the optical scanner.
Ideally, a rotary position detector should only be sensitive to the rotational angle of the scanner shaft. Since a mirror is connected directly to the scanner shaft, it is the rotation angle of the shaft that dictates the direction of the exiting light beam. Axial motion and radial motion generally will not affect the target position of the light beam being reflected by the mirror, and since it is the target light beam position that is important to the scanning system, the output of the position detector should indicate the target position, and be insensitive to things that do not affect that target position, such as axial and radial motion. Axial shaft motion may occur as a dynamic behavior of the scanner. For example, if the magnetic construction of the scanner is not perfect, the shaft may surge outward or inward when strong current pulses are put into the scanner during strong acceleration and deceleration. Radial motion of the scanner can occur as a result of bearing “rumble” or imperfections in manufacturing, which allow a small amount of radial motion of the shaft. Radial shaft motion can also occur as a dynamic effect, if the rotor is not perfectly concentric with the stator components, or if the inertial load (mirror and mount) attached to the output shaft is not perfectly balanced.
Typically, a servo controller is connected between the position detector and the motor. If the position detector produces some output as a result of axial or radial shaft motion, the servo controller will mistakenly interpret this errant output as a change in rotational position, resulting in a positioning error of the overall system. For that reason, a perfect rotary position detector will produce an output only as the result of rotational motion, and will not produce an output as the result of axial or radial motion.
An additional desirable property of a rotary position detector, especially for galvanometer scanners used with analog servo systems, includes the feature that the output voltage or current be linear with respect to the rotation angle. That is to say, an incremental change in shaft rotation should produce an equally incremental change in output signal from the position detector, although a small degree of nonlinearity is often tolerable. Further, the signal-to-noise ratio should be as high as possible.
There are several ways to sense the position of the shaft within an optical scanner. Two popular types of position detectors comprise capacitive position detectors and optical position detectors.
Capacitive position detectors were used in some of the very earliest galvanometer-based optical scanners. In one known detector, a rotating dielectric butterfly is connected to the scanner shaft, and the detection plates are fixed.
Optical position detectors have emerged recently as the position detector of choice in the field of galvanometer-based optical scanning. Typically, optical position detectors can be made small, and have low inertia, and can be manufactured at low cost. These properties make optical position detectors desirable for optical scanners applied in commercial and consumer markets.
One type of optical position detectors is a “shadow cast” position detector, wherein a large area of light sensor material is attempted to be evenly illuminated, and a shadow is cast on the light sensors by a light blocker. Optical position detectors can use photocells as the light sensors. These photocells are most commonly bulk-area PIN photodiodes, and are used in the “photovoltaic” mode, whereby an electrical current is produced by the photocell, and amplified by an op-amp. The amount of electrical current increases linearly as the intensity of the light over the entire area of the photocell increases linearly. The amount of electrical current also increases linearly as the illuminated portion of the photocell is linearly increased, as long as the illumination across the entire area is constant. That is, if light is illuminating half the light sensor area, and light is blocked from the other half of the light sensor area, the electrical current that is output will be half the amount as of that for a complete illumination of the light sensor, yielding a linear relation of position detector output to photocell area illumination. Many existing “shadow cast” detectors, however, often suffer from undesirable axial and radial shaft movements as described hereinabove.
Position detectors used in optical scanning systems are generally well known, as described by way of example in U.S. Pat. Nos. 7,688,432; 7,940,380 and; 8,508,726, the disclosures of which are herein incorporated by reference in their entirety and commonly owned.
Moreover, as described in U.S. Pat. No. 5,844,673, the disclosure of which is herein incorporated by reference in its entirety, a typical square chip approach is currently very common.
However, it would be beneficial to provide a rotary position detector capable of accommodating a shaft having a diameter greater than that of preexisting devices, thereby reducing undesirable shaft movements and thereby improving signal-to-noise ratio and position accuracy. The embodiments disclosed herein are aimed at overcoming these and other needs in the art.